The present invention relates to formations penetrated by wells which are experiencing water influx due to casing problems, coning, channeling, fingering, or any other reservoir related cause of water influx. More particularly it relates to a method for the identification of the precise point of water influx.
It also relates to downhole video technology, to water influx identification using downhole video equipment, and to a method of replacing the clear fluid in the wellbore prior to video monitoring procedures with a fluid which is clear yet provides sufficient contrast to better facilitate the precise identification of water influx points in the well.
Oil field operators must frequently contend with the problem of excessive water influx in producing wells and of poor distribution profiles in water injection wells. Water shutoff represents an enormous expense in the oil industry. The major problem in performing water shutoff jobs is identifying the precise points of water influx.
In the production of hydrocarbons from a hydrocarbon-bearing formation there is normally provided a well which extends from the surface of the earth into the formation. The hydrocarbon-bearing portion of the formation may be overlain or underlain by a water-bearing portion of the formation.
The well may be completed by employing conventional completion practices such as running a cement casing in the well and forming perforations through the casing and cement sheaths around the casing, thereby forming an open production interval which communicates with the formation.
In the case of a hydrocarbon-bearing formation it is normally desirable to form the open production interval so that it communicates with the oil-bearing portion but does not extend into and communicate with the water-bearing portion. However, the open production interval which is formed in the well may inadvertently communicate with a water-bearing portion which is completed in the same wellbore as the hydrocarbon-bearing portion of the formation.
Even if there is no actual initial fluid communication between the open production interval and the water-bearing portions of the formation, such communication may develop during production of hydrocarbon from the hydrocarbon-bearing portion of the formation. For example, water may be drawn upwardly from the water-bearing portion into the oil-bearing portion about the well. This phenomenon is known as water coning. In the case of water coning, free water is produced in the well which results in a much higher water-to-oil ratio in the production stream than would be the case without the water coning.
A phenomenon called fingering can also occur where the viscosity of one fluid, such as water, causes the development of fingers or bulges which may be caused by points of minute heterogeneities in the reservoir. These fingers of displacing fluid tend to become extended in the direction of flow.
In situations where secondary recovery of oil is being accomplished by water flood, it is frequently found that areas of high permeability exist at points along the interior of the well into which flood water is being injected. Instead of providing the desired uniform sweep through the formation, the flood water channels through zones of high permeability, xe2x80x9cthief zonesxe2x80x9d, and finds its way to a producing well without having served any useful purpose.
Casing problems are also quite common. In the case of an oil well, for example, after the steel casing or tubing has been in place for some time, rusting and occasional shifts in the earth will cause rupturing or uncoupling of the steel casing. Water can then enter the well at these points. When this happens, visual examination is necessary to identify precise point of water influx, the extent of the break or leak, and the feasibility of repairs. Accordingly, the visual examination of the walls of a well is frequently needed when applied to the above problems.
In other instances, exploration holes are drilled to locate mineral deposits such as oil and gas, ground water, and geothermal supplies, to check the integrity for nuclear waste depositories, and also to determine the potential for landslides in an unstable environment. In any of these situations it is often possible for a crack or rupture to allow the influx of water.
Methods are known in the art for monitoring wells and for locating and analyzing fluid influxes.
In U.S. Pat. No. 4,980,642, incorporated herein by reference in its entirety, there is described a method of detection of influx of fluids invading a borehole.
U.S. Pat. No. 5,070,949, incorporated by reference in its entirety, discloses a method of controlling a well drilling operation and monitoring drilling parameters to detect a fluid influx.
Closed circuit TV camera systems are also known in the art for visually examining the walls of a given borehole. In large diameter boreholes, a trained geologist can be physically lowered into the hole with a light source to visually examine the stratification, fracturing, and layering of the various geological formations down to which the borehole penetrates. In smaller diameter holes, this type examination is impossible. Accordingly, in smaller holes visual wall examination must be made with a moving picture borehole camera or with a closed circuit television video camera. Additionally, the bore shaft itself made by the borehole is often not in a vertical orientation and has a drift or deviation in azimuth from its true vertical. There are drift recorders which monitor and log the slanting or drifting of the borehole from its true azimuth. Inclinometers are known which determine deviation as well as drift, for example, by photographing from a plumb bob position against a compass background.
U.S. Pat. No. 4,855,820 describes an apparatus and method of visually examining the sidewalls of a borehole, including a downhole video tool lowered into the borehole by means of a cable and winch on the surface. The apparatus includes a wide angle video camera enclosed in its lower section. An upper section houses, for example, a power supply/triplexer, a telemetry board, an FM modulator video amplifier transmission board, gyro data interface board and a gyroscope for showing the directional orientation of the camera and apparatus in the borehole. The gyroscope orientation and the visual image of the portion of the sidewall viewed is transmitted to a video display monitor in an equipment van on the surface. The image on the screen includes a directional reference point so that the direction of a portion of the sidewall being viewed can be ascertained. The camera images are recorded by a video cassette recorder for a permanent record of the visualization of the entire length of the borehole. Various geological data can be extrapolated by this visualization by means of the fracturing and stratification which may be observed in a given borehole. Additionally, the probe can be used to inspect boreholes previously encased by steel tubing to detect any leaks or other deterioration in the tubing system.
Positively identifying the precise source of water influx is a key step in being able to successfully treat the problem. Furthermore, for video monitoring to be successful, clear fluids must be present across and adjacent to the targeted viewing interval. This creates a problem when trying to determine the point(s) where water entry is occurring because there is no contrast in the fluids, making it almost impossible to define water flow. That is, water is entering into a water phase. When exploration personnel review video tapes of actual footage recorded downhole, the lack of contrast in fluids when looking for water entry is very evident, especially at reduced producing rates. Thus far, no practical solution to the problem is known in the art. In actual situations in the field the lack of contrast in water at the point of entry negates the value of any information obtained by downhole video cameras. Logs obtained in situations such as described cannot be accurately interpreted. Based on the attempted interpretation of such logs, the perforations could have been either producing water or completely plugged and not contributing. Without some type of indication technique to better identify the water source the success ratio of using this tool as a diagnostic is greatly reduced.
With these problems in mind, it would constitute a distinct advance in the art if there were an effective, low cost method of treating fluid used to analyze water influx which would make the precise point of water influx much easier to detect.
In accordance with the foregoing the present invention is directed to any method known in the art for identifying water entry points in a well and to the improvement comprising: Prior to the steps for locating the water influx points, replacing the wellbore fluid with a fluid selected from:
a) water to which has been added a viscosifying agent, a coloring agent and, optionally, sufficient salt to increase the density; and in the alternative, a fluid comprising
b) a transparent water-insoluble solvent.
Either of these two fluids provide a distinct contrast between the injected fluid and the wellbore fluid and provide an improvement over anything presently available in the art.
In the first embodiment it has been discovered that neither the viscosifying agent nor the coloring agent alone sufficiently solves the problem, but with the two in combination it is possible to observe a distinct contrast. In the second embodiment the water-insoluble solvent enters the wellbore as small bubbles.